Interpretive Summary: Accurate prediction of the behaviour of pesticides in the environment relies on robust understanding of their interactions with soil. Sorption is a key process in soil that influences pesticide efficacy as well as availability for runoff, leaching, degradation and uptake into non-target organisms. Here, we propose a three-site model to re-analyse the experimental data. The model adds a slow but reversible binding on non-equilibrium sorption sites in addition to instantaneously reversible sites and irreversible sites. The model was able to match experimental data very closely, but only if irreversible sorption was assumed to be absent. In particular, asymmetry in the binding of 12C- and 14C-pesticide was explained on the basis of non-attainment of sorption equilibrium over the study period. Results suggest that truly irreversible sorption may be less significant than previously considered with important implications for understanding the fate of pesticides in applied to soil. The new information obtained using this approach, effectively confirms the need for scientists to include time-dependent sorption in sorption modelling.

Technical Abstract:
Previously published research has used an isotope-exchange technique to measure irreversibility of pesticide adsorption-desorption in soil. Results have indicated significant irreversibility (6-51%) in the sorption in five pesticide-soil systems measured over 72 hours. Here, we propose a three-site model to re-analyse the experimental data. The model adds a slow but reversible binding on non-equilibrium sorption sites in addition to instantaneously reversible sites and irreversible sites. The model was able to match experimental data very closely, but only if irreversible sorption was assumed to be absent. In particular, asymmetry in the binding of 12C- and 14C-pesticide was explained on the basis of non-attainment of sorption equilibrium over the study period. Results suggest that truly irreversible sorption may be less significant than previously considered with important implications for understanding the fate of pesticides in applied to soil.